What computer operating system are you running your Premiere Elements 12 on? Are you working with a NTSC or PAL setup?
Assuming NTSC setup...
You can set a Still Image Default duration so that all the stills that are imported after that set import with the same duration (the default is 150 frames equivalent to 5 seconds in a 30 frames per second system) See Expert workspace, Edit Menu/Preferences/Still Image Default duration.
Once you get your stills on the Timeline (at the preferences set), if you want to, you can change the Still Image duration for one or more of the stills on the Timeline. To do that for one, right click the Timeline still, select Time Stretch, and , then in the Time Stretch dialog set the duration that you want. There the format is 00;00;00;00 (hours;minutes;seconds;frames). So, a 3 second duration set would look like 00;00;03;00.
If you wanted to do the Timeline still duration change on more than one at a time basis, then adjust the first, followed by a copy and paste effects and adjustments in the selected rest.
Please let us know if you are Ok with the details or you need an clarification on anything written.
Add On...If you are developing a workflow for a Timelapse video in Premiere Elements, please let me know.
I have some step by step how tos on that.